The preparation of PCTFE is well known in the art, as well as are PCTFE-based copolymers that include, but are not limited to, CTFE-vinylidene fluoride, CTFE-tetrafluoroethylene, and CTFE-ethylene copolymers. These materials are described in detail, for example, in the ENCYCLOPEDIA OF POLYMER SCIENCE AND ENGINEERING, Sec. Ed. Vol. 3, at page 463 (Pub. John Wiley and Sons). As therein described, articles and films formed from PCTFE materials are non-flammable, resistant to chemicals and strong oxidizing agents, and exhibit desirable moisture barrier properties.
There are presently known a variety of processes that are suitable for the formation of CTFE-based polymers. High molecular weight homo and copolymers of CTFE may be prepared by free radical-initiated polymerization either in bulk, suspension, or aqueous emulsion via the use of a suitable initiator system or in the alternative by ionizing radiation.
PCTFE formed by an aqueous suspension process employs redox initiators such as alkaline metal persulfates and bisulfites with iron, copper, or silver salts as catalysts under acidic conditions (pH≦2.0). The PCTFE polymer produced by this method is acidic and exhibits poor thermal stability during processing at temperatures from about 275° C. to about 325° C. The poor thermal stability is attributed to encapsulation of the polymer with inorganic moieties from the initiating species at one or both ends. These ionic end groups may undergo hydrolysis during work-up to form unsaturated olefins and carboxylic acids. Thus, thermally-pressed plaques prepared from PCTFE samples produced by the above method often undesirably show bubbles and discoloration, which is thought to be due to low molecular weight oligomers formed at the end of the polymerization when the reaction is taken to high conversion. Acidic PCTFE polymers extract metals from the reactors to form metal salts during work-up. Encapsulation of inorganic moieties produce PCTFE resin with a relatively high content of residual ash, which limits the range of applications within which articles formed using the PCTFE polymer resin may be used. Such polymeric materials are known to have lower dielectric strengths, which often render them undesirable for use in electrical and electronic devices and/or packaging.
PCTFE polymers produced by the emulsion polymerization method require a surfactant to form a stable emulsion. Most surfactants are fluorinated compounds with a polar head group, and removal of the surfactant is an important part of the work-up process. Complete removal is often very difficult depending on the extent of adsorption to the polymer particles. Further, recent studies indicate that these surfactants are bioaccumulable, toxic, and environmentally persistent.
Regardless of the method employed, PCTFE homopolymer has a strong tendency to crystallize and thus molecular weights are kept high to maintain the degree of crystallinity for optimum physical, mechanical, electrical, and barrier properties. Degradation of the polymer during processing and polymer acidity should be reduced for most end use applications. This may be achieved by neutralizing the PCTFE polymer with a suitable neutralizing agent such as a buffer solution or an aqueous base.
Therefore, it will become apparent to those skilled in the art that there remains a present and continuing need for improved PCTFE homo and copolymers that are suitable for the production of articles therefrom when processed at elevated temperatures, or to be included in the structure of the formed article. There also remains a continuing need in the art for the production of thermally stable, neutral PCTFE homo and copolymers that have improved machine processability, particularly with regard to melt-extrusion, pelletization, thermoforming, and lamination. Still further, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.